In both deuterostomes and protostomes, a zygote first develops
into a hollow ball of cells, called a blastula. In deuterostomes, the early
divisions occur parallel or perpendicular to the polar axis. This
is called radial cleavage, and also occurs in certain
protostomes, such as the lophophorates. Most
deuterostomes display indeterminate
cleavage, in which the developmental fate of the cells in the
developing embryo are not determined by the identity of the parent
cell. Thus if the first four cells are separated, each cell is
capable of forming a complete small larva, and if a cell is removed
from the blastula the other cells will compensate.

In deuterostomes the mesoderm forms as evaginations of the
developed gut that pinch off, forming the coelom. This is called enterocoely.

Both the Hemichordata and Chordata have gill slits, and primitive fossil echinoderms
also show signs of gill slits. A hollow nerve cord is found in all
chordates, including tunicates (in the larval stage). Some
hemichordates also have a tubular nerve cord. In the early
embryonic stage it looks like the hollow nerve cord of chordates.
Because of the degenerated nervous system of echinoderms, it is not
possible to discern much about their ancestors in this matter, but
based on different facts it is quite possible that all the present
deuterostomes evolved from a common ancestor that had gill slits, a
hollow nerve cord and a segmented body. It could have resembled the
small group of Cambrian deuterostomes named Vetulicolia.

Formation of mouth and
anus

Deuterostome means "secondary mouth", and related to the fact
that after the anus forms, a secondary opening forms in
deuterostome embryos that goes on to be the mouth; the gut tunnels
down from the mouth to anus to connect the two.

Origins

The majority of animals more complex than jellyfish and other Cnidarians are split
into two groups, the protostomes and deuterostomes, and chordates
are deuterostomes.[2]
It seems very likely that 555 million years
oldKimberella was a member of the
protostomes.[3][4]
If so, this means that the protostome and deuterostome lineages
must have split some time before Kimberella appeared — at
least 558 million years
ago, and hence well before the start of the Cambrian 542 million years
ago.[2]
The Ediacaran fossil
Ernietta, from
about 549 to 543
million years ago, may represent a deuterostome animal.[5]

Fossils of one major deuterostome group, the echinoderms (whose modern
members include sea stars,
sea urchins and crinoids) are quite common from
the start of the Cambrian, 542 million years
ago.[6]
The Mid Cambrian fossil
Rhabdotubus
johanssoni has been interpreted as a pterobranch hemichordate.[7]
Opinions differ about whether the Chengjiang fauna
fossil Yunnanozoon, from the earlier
Cambrian, was a hemichordate or chordate.[8][9]
Another Chenjiang fossil, Haikouella
lanceolata, also from the Chengjiang fauna, is interpreted
as a chordate and possibly a craniate, as it shows signs of a
heart, arteries, gill filaments, a tail, a neural chord with a
brain at the front end, and possibly eyes — although it also had
short tentacles round its mouth.[9]Haikouichthys and Myllokunmingia, also from the
Chenjiang fauna, are regarded as fish.[10][11]Pikaia, discovered
much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a
primitive chordate.[12] On
the other hand fossils of early chordates are very rare, since
non-vertebrate chordates have no bones or teeth, and none have been
reported for the rest of the Cambrian.